In plumbing installations, copper tubing is widely employed. In risers, used for connecting tubing to fixtures or tanks, the end of the copper tubing is shaped to form a bulb sealing surface and such bulb includes a shoulder permitting the tubing and thus the bulb sealing surface to be drawn into biting or sealing engagement with the fixture. The cost of such copper tubing and the cost of forming the same to permit the connection to such fixtures or tanks is substantial.
More recently, polybutylene was approved for use in plumbing. Tubing or pipe made of polybutylene is normally joined by heat-fusion techniques, by mechanical compression, and by cold flaring. In order to provide such polybutylene tubing with a bulb sealing surface or an end cap for such purposes, a variety of techniques have been employed. Two commonly employed techniques are: (1) spin-welding a separately molded bulb onto the outer diameter (O.D.) of the end of a tube; or (2) insert molding a bulb onto the O.D. of the end of a tube. All such processes have cost and performance drawbacks. Most require separately molded parts which must be joined to the tubing in assembly operations. Moreover, a two-part tubing end cap or bulb sealing construction does not have the performance integrity or the expected useful life of the tubing itself. In the spin welding technique, excessive clamping pressures may cause the loaded part to become dislodged or separated from the O.D. of the tubing and the interface of the parts provides a possibility of leakage. In the case of a neoprene or like washer employed on the O.D. of the tubing, the same interface leakage susceptibility is present. Moreover, a flange formed to receive the washer may itself create a point of weakness if excessive clamping pressures are employed. Further neoprene washers are known to deteriorate with age and temperature exposure. Lastly, insert molding forces hot material over a cold tube surface, which can separate from the tube.
The solution to this problem of providing polybutylene tubing with an attached bulb sealing surface of unitary construction is detailed in U.S. Pat. Nos. 4,316,870, 4,446,084 and 4,525,136. The thrust of these references however, is to teach the ability to maintain a constant diameter opening within the tubing, while the wall thickness is variable. This is of necessity, due to the configuration of the mold cavity, and insertion of the mandril inside the tubing during the processing steps.
A corresponding associated problem with the formation of the above-described male end of the polybutylene tubing, is the ability to bell an opposed end of the tubing, without any accompanying wall thickness compromise, which would make the product unsuitable for all plumbing applications, for which polybutylene has been approved, provided that a wall thickness can be maintained at 0.062"+0.010", as defined by ASTM 3309. In particular, it is desirable to use 3/8" O.D. polybutylene tubing with wall thickness of 1/16" (0.062") and subsequently insert a 1/2 " CTS (copper tube size) fitting of nominal 0.501" O.D. The only way this can be achieved is through belling one end of the tubing from 3/8" O.D. (1/4" I.D.) to 5/8" O.D. (1/2" I.D.). While it is possible to use 5/8" O.D. tubing to start, this uses more raw materials than necessary.
Prior art solutions to the formation of a bell on one end of polybutylene tubing is by heating a portion of the end of the tubing, followed by insertion of a mandril into the heated open end, the O.D. of the mandril being matched to the targeted inner diameter (I.D.) of the tubing. While this approach will bell the tubing, it is incapable of reproducibly making tubing product with a constant wall thickness of 0.062"+0.010" throughout the belled end, particularly in the neck region of the bell. This is due to the fact that the bell is made by expanding the I.D. and thus thinning the walls. A solution to this problem is found in U.S. Pat. No. 5,527,503.
The trend today however, is to shift from thermoplastic materials, e.g., polypropylene, polybutylene, etc., to thermoset materials, e.g., crosslinked polyethylene. However, this shift in materials is not simple in that there are several processing changes which must be incorporated in order to fabricate acceptable parts. Since thermosets cannot be extruded like thermoplastics, differing processing conditions must be employed in different sequences in order to achieve similar functionality for the product. For example, it is not possible to simply take a crosslinked polyethylene tube and mold it into a bulb end by taking the polybutylene technology taught in the prior art. Previously crosslinked material will not chemically bond to itself even when heated to the clear state. This means that the material in the formed ends is not completely sealed upon itself, but rather molded in place with pressure. One prior art solution to this problem is the use of metal inserts which are positioned into crosslinked polyethylene tubes and subsequently crimped in order to achieve a fitting. This is an inherent weak spot in the final product, and the industry has long sought to find a solution to the problem of developing a one-piece plumbing part made out of a thermoset plastic.